Electroacoustic transducer



;lU-;5J4 5 XR 3,222,462 J g g; a7;

R- KARMANN ET AL ELECTROACOUSTIC TRANSDUCER Dec. 7, 1965 Filed Sept. 26,1962 small thickness of the plate.

Claims. in. 179-110 The present invention relates to an electroacoustictransducer having as a diaphragm a flat vibrating plate which comprisesat least one layer of electrostrictive material.

Transducers of this type have been known for a long time. However, theyhave not achieved any importance in actual practice since theirsensitivity is too low as compared with the sensitivity ofelectroacoustic transducers of conventional type. It has already beenattempted to increase the sensitivity of such transducers by taking intoconsideration the form of vibration caused in case of firm clamping ofthe vibrating plate, by means of a special arrangement of theelectrodes, namely, an interruption thereof at the points of change ofthe direction of curvature of the diaphragm, and in addition, byreducing the thickness of the plate to less than 0.01 millimeter inorder better to match the diaphragm to the characteristic impedance ofthe surrounding acoustic medium, by applying the electrostrictivematerial by volatilization to a thin support. Such transducers. however,likewise have not gained acceptance up to now. In particular, theapplication of a homogeneous electrostrictive layer in the necessarythickness, by volatilization, produces considerable difficulties from amanufacturing standpoint.

The present invention shows how the sensitivity of electrostrictivetransducers of the initially noted type can be increased without itbeing necessary to reduce the thickness of the plate to such an extentthat it becomes difficult to manufacture.

Theobject of the invention is realized by smoothing the surface of theelectrostrictive material which lies below the contact material, and byholding the vibrating plate at the edge between a fixed support and aresilient support.

This arrangement is based upon recognition of the fact that it is notsutlicient for the electrostrictive material merely to be ground,particularly in the case of a The scorings produced by the grindingcause a crowding of the lines of force at the notch-like interruptionsin the electrostrictive material, upon the polarization of the material,thus resulting in an excessive heating at these points. Upon unlimitedpolarization current, this will lead finally to a disruptive discharge,while with limited polarization current, the voltage breaks down betweenthe electrodes of the electrostrictive material so that it is no longersufficient to impress a sufliciently high polarization field strength inthe rest of the electrostrictive material.

It is for sufiicient polarization important, in addition to ahomogeneous electrostrictive material and a uniform thickness of theplate, to provide as smooth as possible a surface of theelectrostrictive material below the electrodes through which thepolarization voltage is applied.

The vibrating plate of the transducer of the present invention comprisespreferably two smooth contacted disks of electrostrictive material whichare about 0.2 millimeter thick and bonded or soldered together. Suchdisks can be made by grinding and subsequent lapping, from thickerplates, which can be produced in a partic ularly homogeneous,stress-free and crack-free manner. They have the advantage overextremely thin plates of electrostrictive material that they are easierto manufacted States Patent 'ice ture, that the resistance againstbreakage is greater, that unavoidable small discontinuities, such asinhomogeneity of the material, inclusions of foreign bodies, haircracks, etc., exert a smaller pcrcentual effect and that the naturalvibration lies in the upper part of the transmission range. Whilematching to the surrounding acoustic medium is better in the case ofextremely thin vibrating plates, the natural vibration lies in the lowerpart of the transmission range. As a result, the transmission rangedrops so greatly in the upper part that upon linearization of thefrequency response, the increase in sensitivity obtained by the bettermatching is necessarily lost.

In the arrangement provided by the present invention, an increase insensitivity is obtained not only by the provision of a surface which isas smooth as possible, namely, having a roughness of not more than 5microns, resulting in increased polarization, but also by the particularnature of the mounting of the vibrating plate. The

mounting is so constructed that the vibrating plate oscillates like aplate which rests freely on its edge. For a given thickness of theplate, this means an increase in sensitivity by about a factor of 3.2,as compared with the fixed mounting used heretofore in electrostrictivetransducers. The lesser stiffness of the resilient clamping also resultsin better matching to the characteristic impedance of the surroundingacoustic medium.

For the desired increase in sensitivity, it is also ad visable that themetal coatings of the electrostrictive disks do not extend up to theedge thereof, but leave an electrode-free edge zone. The form ofoscillation which occurs with the mounting of the diaphragm inaccordance with the invention, can thereby be taken into consideration.It was found in this connection that the sensitivity is greatest with adiameter of the metal coatings equal to about 0.91 times the diameter ofthe annular fixed support. A combination of all the aforementionedmeasures results in a sensitivity which reaches a value correspondingapproximately to that of the sensitivity of electroacoustic transducersof the traditional type.

The invention will be explained in further detail with reference to theaccompanying drawing.

FIG. 1 shows part of an electroacoustic transducer which is of interestin connection with the invention;

FIG. 2 shows a modified mounting for the diaphragm; and

FIG. 3 shows a further modified mounting for the diaphragm.

In FIG. 1, numeral 1 indicates the housing of the electroacoustictransducer which is closed by a cover- 2. The transducer has aplate-shaped diaphragm 3 which comprises two layers 4 and 5 ofelectrostrictive material and is held between a rigid support 6 and aresilient support 7. The rigid support is made in the form of aprotrusion of trapezoidal cross-section in the wall of the housing. Theresilient support is made as a ring of resilient material, such as softrubber, foam, thermoplast, etc. The cross-section of this ring has inthis case also a trapezoidal shape. However, it could just as well berectangular, since it is to consist of such a soft material that it willnot interfere with the. movement of the diaphragm regardless of itsresting surface against the diaphragm. The two electrostrictive layersof the diaphragm are in each case metallized on both sides and bonded orsoldered together. In order that the desired movement of the diaphragmbe obtained, the two electrostrictive layers are polarized axially inthe same direction and the two metal coatings 8 and 9 which lie againsteach other and the outer coatings 10 and 11 of the diaphragm are in eachcase connected with each other and connected with an electricalterminal. The metal coatings do not cover the entire surface of theelectrostrictive material but only about nine-tenths of the diameter ofthe annular stationary support. The edge is thus maintained inactive.

In the embodiment shown in FIG. 2, the stationary support 12 is made inthe form of a sharp-angled protrusion with rounded edge in the wall ofthe housing. The opposite support consists of a resilient metal ring 13which is made in desired known manner and rests against the housingcover or is fastened to it.

In the embodiment shown in FIG. 3, the stationary support is made in theform of a non-yieidable metal ring 14 of circular cross-section, whilethe resilient support is made in the form of a soft rubber ring 15.

Changes may be made within the scope and spiritof the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

We claim:

, 1. An electroacoustic transducer having a diaphragm in the form of avibrating plate, said plate comprising at least one disk ofelectrostrictive material, a layer of a con- 7 ductive contact materialdisposed on eachface of the disk,

the surface of the electrostrictive material disposed beneath suchcontact material having a roughness of at most about 5 microns, andmeans for supporting said vibrating plate at its edge, said means havinga stationary supporting element and a resilient supporting element,between whichelements said plate is disposed.

2. An electroacoustie transducer according to claim 1, wherein the diskof electrostrictive material is about 0.2 millimeter thick and isprovided with ground and lapped surfaces.

3. An electroacoustic transducer according to claim 1, wherein thevibrating plate consists of two electrostrictive circular disks whichare metallized on both sides and bonded together, and wherein thediameter of at least the two outer metal coatings of the vibrating plateis less than thediameter of the stationary support thus leaving acoating-free outer edge.

4. An electroacoustic transducer according to claim 3, wherein thediameter of at least the two outer metal coatings of the vibrating plateis about 0.91 times the diameter of the annular stationary support.

5. An electroacoustic transducer according to claim 4, wherein thestationary support is part of the housing wall in the form of aprojection of trapezoidal cross-section.

6. An electroacoustic transducer according to claim 4, wherein thestationary support is part of the housing wall in the form of anacute-angled projection with rounded edg 7. An electroacoustictransducer according to claim 4, wherein the stationary support is aring of hard material, said ring being disposed upon a ledge of thehousing wall.

8. An electroacoustic transducer according to claim 7, wherein theresilient support is a ring of resilient material, selected from theclass of'materials consisting of soft rubber, foam material. thermoplastand the like.

9. An electroacoustic transducer according to claim 6, wherein theresilient support is a metal ring having resilient properties.

10. An electroacoustic transducer according to claim 9, wherein saidmetal ring is fastened to the housing cover.

References Cited by the Examiner Electronic Industries" magazine,February 1959, p. 72. Radio-Electronics magazine, September 1959, pp.30-31.

ROBERT H. ROSE, Primary Examiner.

1. AN ELECTROACOUSTIC TRANSDUCER HAVING A DIAPHRAGM IN THE FORM OF AVIBRATING PLATE, SAID PLATE COMPRISING AT LEAST ONE DISK OFELECTROSTRICTIVE MATERIAL, A LAYER OF A CONDUCTIVE CONTACT MATERIALDISPOSED ON EACH FACE OF THE DISK, THE SURFACE OF THE ELECTROSTRICTIVEMATERIAL DISPOSED BENEATH SUCH CONTACT MATERIAL HAVING A ROUGHNESS OF ATMOST ABOUT 5 MICRONS, AND MEANS FOR SUPPORTING SAID VIBRATING PLATE ATITS EDGE, SAID MEANS HAVING A STATIONARY SUPPORTING ELEMENT AND ARESILIENT SUPPORTING ELEMENT, BETWEEN WHICH ELEMENTS SAID PLATE ISDISPOSED.